A brake is a device for applying a force against the friction of the road, slowing or stopping the motion of a machine or a vehicle, or a device to restrain such a machine or a vehicle from starting to move again. Brakes of some description are fitted to most wheeled vehicles, including automobiles of all kinds.
In an automobile, a braking system is employed for slowing or stopping the rotation of a road wheel by converting kinetic energy into heat, typically via friction. In most automotive braking systems, the brake acts on a hub of the vehicle's road wheel, but some vehicles use brakes which act on the axles or transmission. Friction brake systems commonly employ either a drum or a disc type brake mechanism.
A drum brake mechanism employs a set of brake shoes that are actuated to press against the inner surface of a rotating drum, typically connected to the hub of vehicle's road wheel, in order to generate friction for stopping the vehicle. The disc brake mechanism employs a set of brake pads mounted on a brake caliper, such that the pads are actuated to grip a rotating brake disc, also typically connected to the hub of vehicle's road wheel, in order to generate friction for stopping the vehicle. Such automotive braking systems are hermetically sealed, and commonly utilize a non-compressible brake fluid to transfer hydraulic pressure from a control unit to actuate the brake mechanism. In such braking systems, a loss of system efficiency and braking power may result from air in the system.